1. Field of the invention
This invention relates to a semiconductor light emitting device, and more particularly to a semiconductor light emitting device having strong directivity of light emission.
2. Description of the prior art
Recently, various kinds of light emitting diode, which are made from compound semiconductor materials, such as InGaAlP and GaAlAs, and of the type of direct transition, have been developed. In particular, InGaAlP diodes emit red to yellow-green light by direct transition so that they are expected to be a light source having high luminous efficiency.
FIGS. 1A and 1B show the schematic structure of a prior art light emitting diode of the kind mentioned above having high luminous efficiency. Especially, FIG. 1A shows the cross-sectional view and Fig. 1B shows the plain view of this diode.
As shown in these figures, the light emitting diode has an n-type GaAs substrate 51. An n-type GaAs buffer layer 52, a reflection layer 53, a first clad layer 54 made of n-type InCaAlP, an active (emittive) layer 55 made of undoped InGaAlP, and a second clad layer made of p-type InGaAlP are formed on substrate 51 in this order. Reflection layer 53 has a multilayer structure which is comprised of about 10 pairs of an n-type GaAs layer and an n-type InAlP layer, each of which overlaps the other alternatively. These two kind of layers, that is, n-type GaAs layers and n-type InAlP layers, have different refractive indices from each other. In addition, an n-type GaAs layer 58 is formed on layer 56 in order to interrupt a current flow. However, an aperture 57 is formed in layer 58 so that the current flows through this aperture. Layer 58 is covered by a p-type GaAlAs layer 59 which is to diffuse an electric current. An ohmic electrode 61 of p-type having an aperture 60 (light window) is formed on layer 59. This aperture 60 is provided in order to release the emitted light out of this device, and therefore, it is placed right over aperture 57 and has the same area as that of aperture 57. On the other hand, an ohmic electrode of n-type is formed on the back side of substrate 51.
In the light emitting diode having the structure mentioned above, an electric current supplied via p-type electrode 61 flows through diffusion layer 59 and clad layer 56 and reaches active layer 55 where light emission takes place. In this case, the current flowing through layer 59 is focused with aperture 57 and then applied on a particular area 63 of layer 55, the area which is right under aperture 57. Accordingly, a large amount of current is concentrated to area 63 and so, strong light emission is obtained from this area.
A part of light energy emitted upwards from area comes out of this light emitting diode through aperture 57 and light window 60. On the other hand, the other part of light energy emitted downwards from area 63 is reflected by layer 53 towards the upper direction and comes out of this diode through aperture 57 and light window 60 without being absorbed by substrate 51. As a result, almost all the light energy can be obtained through window 60, and so, a light emitting diode having high luminous efficiency is obtained.
However, the prior art light emitting diode has the following disadvantage. In this diode, most of the current applied through electrode 61 is concentrated to area 63 of active layer 55, the area 63 which is right under aperture 57. This permits the device to emit strong light in area 63. At the same time, however, a small amount of the current leaks out from area 63 of active layer 55, thus permitting weak emission in the circumference of this area. The emitted light from the circumference of area 63 comes out of this device through both sides of layer 55 directly or indirectly being reflected by reflection layer 53, as shown by arrows 64 in FIG. 1. As a result, light emission appears not only in the direction perpendicular to window 60 but in the lateral direction of this device.
When the light emitting diode mentioned above is used in a camera as a light source for printing dates on a film, the light emitted in the lateral direction of this diode reaches the film to expose, in addition to the light obtained through window 60. As a result, the resultant image on the film is not focused and it loses sharpness.
As explained above, the reflective layer of the prior art light emitting diode is formed to cover the entire surface of the active layer, and the emitted area is restricted to a particular area of the active layer due to the existence of an aperture provided in the current stopping layer. Therefore, weak emission, which is produced by a leakage current in the circumference of the emitted area, comes out through both sides of this device directly or indirectly being reflected by the reflective layer. In other words, light emission is seen from this diode not only in the main direction of emission but in the direction perpendicular to the main direction.
According to the above mentioned reason, the directivity of light emission of the prior art light emitting diode is not so strong. Therefore, a sharp image is not obtained when the diode is used as a light source. This is emphasized especially in the case where the target to be imaged by this light emitting diode is small.